3-Iodophenoxyacetic acid

The oxidation of phenols or o- and p-hydroquinones with stoichiometric [bis(acyloxy)iodo]arenes to the corresponding benzoquinones is one of the most typical synthetic applications of hypervalent iodine reagents (Section 3.1.11). The catalytic version of this reaction was first reported by Yakuraand Konishi in 2007 [52], The reaction of p-alkoxyphenols 50 with a catalytic amount of 4-iodophenoxyacetic acid in the presence of Oxone as the terminal oxidant in aqueous acetonitrile at room temperature affords p-quinones 51 in high yields (Scheme 4.26) [52]. 4-Iodophenoxyacetic acid is a readily available and water-soluble aromatic iodide that has a particularly high catalytic activity in this reaction. 

Several modifications of the original procedure shown in Scheme 4.26 have been reported [54-56]. In particular, the reaction of p-dialkoxybenzenes 52 with a catalytic amount of 4-iodophenoxyacetic acid in the presence of Oxone as a co-oxidant in 2,2,2-trifluoroethanol-water (1 2) gives the corresponding p-quinones 53 in excellent yields (Scheme 4.27) [54, 56]. The same solvent system, 2,2,2-trifluoroethanol-water (1 2), can also be used for the efficient oxidation of p-alkoxyphenols to p-quinones [55]. 

A similar catalytic oxidation of p-substituted phenols bearing an alkyl or aryl group in the para position affords the corresponding p-quinols. In particular, the reaction of p-substituted phenols 54 with a catalytic amount of 4-iodophenoxyacetic acid and Oxone (4 equiv) at room temperature in an aqueous tetrahydrofuran or 1,4-dioxane solution gave p-quinols 55 in generally high yields (Scheme 4.28) [55,57]. 

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